1. Field of the Invention
The present invention relates to a communication system in which a terminal (transponder) that does not have its own radio wave generating source transmits data to a host (reader/writer) in a noncontact manner, and a communication device, and particularly to a communication system of a reflected wave transmission (backscatter) type including a reflector (transponder) for transmitting data by a reflected wave formed by subjecting a non-modulated wave to a modulation process and a reflected wave reader (reader/writer) for reading the data from the modulated reflected wave signal from the reflector, and a communication device.
2. Description of the Related Art
More specifically, the present invention relates to a communication system and a communication device that increase the speed of reflected wave transmission without widening a frequency band, and particularly to a communication system and a communication device that increase the speed of reflected wave transmission by increasing the number of levels of reflected waves.
A noncontact communication system referred to as RFID (Radio Frequency IDentification) is known as a communication system that does not have its own radio wave generating source and which transmits data by radio. While other names of RFID include an “ID system,” a “data carrier system,” and the like, the universally common name is an RFID system, or RFID for short. RFID means “a recognizing system using high frequencies (radio).” An RFID system is composed of a transponder referred to also as a tag and a reader/writer for accessing the transponder. The transponder passively operates using a radio wave from the reader/writer side as an energy source. The reader/writer reads information stored within the transponder, and writes information to the transponder.
Noncontact communication methods in the RFID system include a capacitive coupling method, an electromagnetic induction method, a radio wave communication method, and the like. In an RFID system of the radio wave communication method among these methods, a transponder has a reflector for transmitting data by a reflected wave formed by subjecting a non-modulated wave to a modulation process, a reader/writer has a reflected wave reader for reading the data from the modulated reflected wave signal from the reflector, and reflected wave transmission referred to also as “backscatter” is performed. When the non-modulated wave is transmitted from the reflected wave reader to the reflector, the reflector superimposes transmission data on the reflected wave of the non-modulated wave by modulating the reflected wave on the basis of an operation of changing antenna load impedance or the like. That is, the reflector side does not need a carrier generating source in data transmission, and thus drives a data transmitting operation with a low consumption. The reflected wave reader side can obtain the transmission data by receiving such a modulated reflected wave and performing demodulation and a decoding process.
The reflector basically includes an antenna for reflecting an incident continuous radio wave, a circuit for generating transmission data, and an impedance changing circuit for changing the load impedance of the antenna in such a manner as to correspond to the transmission data. The impedance changing circuit is for example an antenna switch for changing the termination of the antenna to an open/ground. While the antenna switch can be incorporated into a communication circuit module and formed by a CMOS (Complementary Metal Oxide Semiconductor) transistor, a high-speed changing operation with low power consumption is made possible by separating the antenna switch from the circuit module and forming the antenna switch by a gallium arsenide (GaAs) IC (Integrated Circuit) In the latter case, a rate of data transmission by reflected wave modulation is improved, and the power consumption is reduced to a few ten μW or less. Therefore, considering a fact that a wireless LAN (Local Area Network) consumes a power of a few hundred mW to a few W at a time of communication, reflected wave communication can be said to have an overwhelming difference in performance as compared with the average power consumption of a common wireless LAN (see for example Japanese Patent Laid-Open No. 2005-064822).
In addition, because the transponder including the reflector performs only an operation of reflecting a received radio wave, the transponder has another advantage of not being regarded as a radio station and not being subject to legal control imposed on radio wave communications. Further, while a noncontact communication system of the past uses frequencies of a few MHz to a few hundred MHz (for example 13.56 MHz), the reflected wave transmission system can achieve high-speed data transmission using a high band of a 2.4 GHz band (microwaves) referred to as an ISM band (Industrial, Scientific and Medical band), for example.
As an example of application of a communication system using reflected wave transmission, there is a case where a reflector as a transponder is incorporated into a terminal as a data source such as a digital still camera, a digital video camera or the like, a host such as a television set, a printer or the like is provided with a function of a reflected wave reader, and data such as still images, moving images or the like is uploaded by reflected wave transmission for display or printout processing.
FIG. 8 shows a configuration of a reflector as a data transmitting source in a reflected wave transmission system. The reflector 10 shown in FIG. 8 includes an antenna 11, an antenna switch 12, an antenna load 13, a band-pass filter 14, and an ASK (Amplitude Shift Keying) detecting section 15. Suppose that a 2.4 GHz band referred to as an ISM band is used for radio wave frequencies.
When the antenna switch 12 is on, the antenna 11 is terminated by the antenna load 13 of 50Ω. When the antenna switch 12 is off, the antenna 11 is open. By such a switching operation, the antenna 11 behaves to be terminated for a non-modulated carrier coming from a reflected wave reader when the antenna switch 12 is on, and behaves to reflect the non-modulated carrier when the antenna switch 12 is off, so that a modulation process can be applied to the reflected wave.
When a communication controlling section 16 receives transmission data generated by a higher-layer application (not shown), the communication controlling section 16 performs on/off operation on the antenna switch 12 connected to the antenna 11 according to the bit image of data. For example, the antenna switch 12 is turned on when the data is 1, and the antenna switch 12 is turned off when the data is 0. That is, the transmission data is transmitted as a reflected wave signal modulated by variations in antenna load impedance which variations accompany the on/off operation on the antenna switch 12.
Incidentally, while the band-pass filter (BPF) 14 and the ASK detecting section 15 are used at a time of receiving an ASK-modulated delivery acknowledgment signal (or data) from the transfer destination, the two blocks are unnecessary when transmission is performed in one direction without transmission delivery acknowledgment.
FIG. 9 shows a hardware configuration of an information device 20 having a reflected wave reader function as a data receiving destination in the reflected wave transmission system.
The information device 20 includes an antenna 21 for a 2.4 GHz band, an antenna switch for alternatively connecting the antenna 21 according to transmitting and receiving operation or a circulator 22 as a substitute for the antenna switch, a receiving section 23, a transmitting section 26, a baseband controlling section 30, a decoding section 31, and an information processing section 32 for performing various arithmetic processing for received data after decoding.
In order to read a reflected wave signal from the reflector 10, a non-modulated carrier for creating a reflected wave needs to be transmitted from the reflected wave reader. In this case, the receiving section 23 includes a quadrature detecting block 24 and an AGC (Automatic Gain Control) amplifier 25. The transmitting section 26 includes a mixer 27 and a power amplifier 28. A frequency synthesizer 29 is further provided.
The transmission of a non-modulated carrier from the transmitting section 26 is achieved by supplying a certain direct-current voltage from the baseband (BB) controlling section 30 to the mixer 27. The frequency of the non-modulated carrier to be transmitted is determined by the frequency of the frequency synthesizer controlled from the baseband controlling section 30. The 2.4 GHz band is used in this case. The non-modulated wave output from the mixer 27 is amplified to a predetermined level by the power amplifier 28, and then sent out from the antenna 21 via the circulator 22.
A reflected wave signal from the reflector 10 has the same frequency as the non-modulated carrier transmitted from the reflected wave reader itself included in the information device 20. The reflected wave signal is received by the antenna 21, and then input to the above-described receiving section 23 via the circulator 22. Because the same local frequency as in transmission is input to the quadrature detecting block 24, an ASK modulating wave applied in the reflector 10 appears in the output of the quadrature detecting block 24. However, the received signal is different in phase from the local signal, and therefore a modulating signal corresponding to a phase difference between the signals appears as an I-axis signal and a Q-axis signal.
The gain of the AGC amplifier 25 is controlled to an optimum value, and output signals of the AGC amplifier 25 are sent to the baseband-controlling section 30. The baseband controlling section 30 performs demodulation from the I-axis signal and the Q-axis signal to digital data. The decoding section 31 decodes the digital data into correct data. The decoded data is thereafter subjected to various processing including reproduction of data contents and storage in the information processing section 32.
When data delivery acknowledgment is to be made to the reflector 10, the baseband controlling section 30 transfers digital data of positive acknowledgement Ack to the mixer 27 when the received packet data is correct, and transfers digital data of negative acknowledgement Nack to the mixer 27 when the received packet data is incorrect. The baseband controlling section 30 then applies ASK modulation to the digital data. Whether the data is correct or incorrect is determined by a CRC (Cyclic Redundancy Check) code added to an image data packet.
In the above-described reflected wave transmitting operation, the reflected wave signal sent out from the reflector 10 is equivalent to an ASK (Amplitude Shift Keying) modulated wave. Other modulating methods for generating a reflected wave include PSK (Phase Shift Keying).
FIG. 10 shows a configuration of a reflector that generates a reflected wave by PSK. Reference numeral 100 denotes an antenna, and reference numeral 101 denotes a high-frequency switch formed by a diode, a gallium arsenide FET (Field Effect Transistor) or the like. One side of a strip line 102 as a λ/4 (where λ is a wavelength being used) phase shifter is open, and therefore when the high-frequency switch 101 is turned on, the antenna 100 is connected to a ground in terms of high frequency. The high-frequency switch 101 is subjected to on/off operation on the basis of transmission data (TX_DATA). The high-frequency switch 101 is controlled to be shorted when turned on, and is controlled to be open when turned off. Thus, by controlling a reflecting end to be shorted and open according to the transmission data, PSK modulation can be applied to the reflected wave of a radio wave arriving at the antenna 100.
However, modulation systems with a relatively low bit rate such as ASK, PSK and the like have a problem in terms of transmission speed.
For example, a communication device has been proposed which includes: a first signal path that provides a first reflected wave by directly reflecting a received radio wave without the radio wave passing through any phase shifter; a second signal path in which a phase shifter for giving a phase difference of λ/4 is connected in series and which provides a second reflected wave shifted in phase by π/2 as compared with the first reflected wave; a third signal path in which a phase shifter for giving a phase difference of λ/2 is connected in series and which provides a third reflected wave shifted in phase by π as compared with the first reflected wave; and a fourth signal path in which a phase shifter for giving a phase difference of 3λ/4 is connected in series and which provides a fourth reflected wave shifted in phase by 3π/2 as compared with the first reflected wave; wherein transmission data is divided into units of two bits, and a phase is assigned to a reflected wave by selecting a signal path corresponding to a combination of 0 and 1 of two bits, whereby QPSK (Quadri Phase Shift Keying) modulation is performed (see for example Japanese Patent Laid-Open No. 2005-136666).
In addition, a communication device has been proposed in which a first to a third phase shifter for giving a phase difference of λ/8 one way are connected in series with an antenna, and which includes: a first signal path that provides a first reflected wave by directly reflecting a received radio wave without the radio wave passing through any phase shifter; a second signal path that provides a second reflected wave shifted in phase by π/2 as compared with the first reflected wave after going and returning through the first phase shifter; a third signal path that provides a third reflected wave shifted in phase by π as compared with the first reflected wave after going and returning through the first and second phase shifters; and a fourth signal path that provides a fourth reflected wave shifted in phase by 3π/2 as compared with the first reflected wave after going and returning through the first to third phase shifters; wherein reflected waves having four phases different from each other by 90 degrees are created according to the values of two data bits, whereby QPSK-modulated reflected waves are created (see for example Japanese Patent Laid-Open No. 2005-136943).
These communication systems in which a QPSK modulation method is adopted in reflected wave transmission can achieve a higher transmission rate of data communication than ASK or PSK.
However, PSK modulation cannot increase transmission speed unless-bandwidth is widened, which is a problem also from a viewpoint of effective use of frequencies at a present time when the depletion of frequencies is regarded as a problem.